Silverleaf (Chondrostereum purpureum) spores infect the tree through wounds. As pruning exposes the tree to wounds, fruit tree growers need to prune correctly to quicken the healing process. The following suggestions will help to decrease the likelihood of silverleaf :

1. Ensure there is enough live wood left around the wound so that a doughnut shaped callus can form in an even ring.

2. Look at the trees pruned last season - if the doughnut callus is not present then pruning has been done at the wrong angle.

3. Remove stubs which can debark and be colonised by the silverleaf fungus.

4. Use the correct pruning tools. Cheap or blunt handsaws, loppers and secateurs will squeeze and split the wood, leaving ragged cuts and causing dieback. Split wood allows spores to enter deep into the xylem tissue. Larger branches (greater than 40 mm diameter) should be pruned using a saw to prevent the wood splitting.

5. Be aware that, unlike some viruses, silverleaf is not transferred on pruning tools. Sterilising pruning tools between uses will not reduce silverleaf.

6. Pay special attention when pruning branches off the trunk as infection of trunk wounds often leads to extensive silvering of the tree.

7. Fruit trees are considered to be susceptible to silverleaf throughout the year. However, more spores are present in winter due to the cool, wet conditions. Therefore, infection during winter pruning is more likely.

8. Try to avoid pruning on calm, damp, overcast days when spore loads are high.

9. Apply wound dressings (paints) to the wounds on the same day you prune, allowing time for the wound to dry. Painting the wounds several days later is a waste of time.

10. Apply the dressing thickly on the wound. Large cuts on the trunk may need a second coat.

11. Never thin the pruning paint with water as the paint will loose its effectiveness. Even though paints are expensive, it is false economy to thin them down.

12. Don’t mix your own wound paint, for example fungicides and acrylic paint. Copper based compounds not only do not work, they increase the tree’s susceptibility to silverleaf.

13. If you don’t have an effective pruning paint, it is better to leave the wound untreated as natural wound fungi will inhibit silverleaf.

14. Don’t apply dressings to wounds with brown staining as they are already infected.

Source:
New for HortNET, August 1996. Written and submitted by Helen Percy from HortResearch fact-sheet, "Protecting Your Trees From Silverleaf" and "Silverleaf Disease, Tree Productivity and Fruit Productivity" (Horticulture News, August 1996. Vol 18 No. 8 : p. 18).